In a normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or other anti-arrhythmia therapies to the heart, via electrodes implanted in contact with the heart tissue, at a desired energy and rate. One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A single-chamber pacemaker delivers pacing pulses to one chamber of the heart, either one atrium or one ventricle. Dual chamber pacemakers are now commonly available and can provide stimulation in both an atrial chamber and a ventricular chamber, typically the right atrium and the right ventricle. Both unipolar or bipolar dual chamber pacemakers exist in which a unipolar or bipolar lead extends from an atrial channel of the dual chamber device to the desired atrium (e.g. the right atrium), and a separate unipolar or bipolar lead extends from a ventricular channel to the corresponding ventricle (e.g. the right ventricle). In dual chamber, demand-type pacemakers, commonly referred to as DDD pacemakers, each atrial and ventricular channel includes a sense amplifier to detect cardiac activity in the respective chamber and an output circuit for delivering stimulation pulses to the respective chamber.
If an intrinsic atrial depolarization signal (a P-wave) is not detected by the atrial channel, a stimulating pulse will be delivered to depolarize the atrium and cause contraction. Following either a detected P-wave or an atrial pacing pulse, the ventricular channel attempts to detect a depolarization signal in the ventricle, known as an R-wave. If no R-wave is detected within a defined atrio-ventricular interval (AV interval or delay), a stimulation pulse is delivered to the ventricle to cause ventricular contraction. In this way, rhythmic dual chamber pacing is achieved by coordinating the delivery of ventricular output in response to a sensed or paced atrial event.
Early pacemakers stimulated the heart at a fixed rate. Rate-responsive pacemakers were developed in order to provide an adaptable stimulation rate responsive to the physical need of the patient. An implanted rate-responsive pacemaker (or implantable cardioverter defibrillator having rate-responsive pacing capabilities) typically operates to maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increases the maintained heart rate in accordance with increases in physical activity until a maximum rate is reached. Thus, such rate-responsive pacemakers typically include processing circuitry that correlates measured physical activity to an appropriate heart rate. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate and the transition rates between the minimum and maximum heart rates are parameters that may be telemetrically adjusted to meet the needs of a particular patient.
Most rate-responsive pacemakers employ sensors that transduce mechanical forces associated with physical activity to determine the level of metabolic need of a patient, relying upon the clinical association of body motion with increasing levels of exercise. These activity sensors generally contain a piezoelectric transducing element that generates a measurable electrical potential when a mechanical stress resulting from physical activity is experienced by the sensor. Thus, by analyzing the signal from a piezoelectric activity sensor, a rate-responsive pacemaker can determine how frequently pacing pulses should be applied to the patient's heart. Reference is made to U.S. Pat. No. 5,514,162 to Bornzin et al.
Other physiological sensors frequently used in rate-responsive pacemakers are respiration sensors. Such sensors may be employed to measure respiratory rate, tidal volume, or the product of these two parameters, minute ventilation. Each of these parameters increases in proportion to changes in carbon dioxide production associated with physical activity. Minute ventilation-sensing, rate-adaptive pacing systems have been demonstrated to provide rate modulation that is closely correlated with oxygen consumption in most patients implanted with these devices. Reference is made to U.S. Pat. No. 5,964,788 to Greenhut.
In dual chamber pacemakers (or implantable cardioverter defibrillators), pacing rate may be based on a ventricular stimulation rate or an atrial stimulation rate with the interval between atrial and ventricular stimulation determined by a programmable atrioventricular delay (AV delay or AV interval). In a healthy heart, the interval between an atrial P-wave and a ventricular R-wave varies as heart rate varies. Thus to provide rate-responsive stimulation that is more physiological, rate responsive pacemakers having an adjustable AV interval have been developed. By adjusting the AV interval, appropriate AV synchrony may be maintained as the pacing rate is varied according to the physiological sensor output. Adjustment of the AV interval can be advantageous in maximizing cardiac output or in ensuring ventricular pacing occurs when pacing control of the heart rate is desired.
Mounting clinical evidence supports the evolution of more complex cardiac stimulating devices capable of stimulating three or even all four heart chambers to stabilize arrhythmias or to re-synchronize heart chamber contractions (Ref: Cazeau S. et al., “Four chamber pacing in dilated cardiomyopathy,” Pacing Clin. Electrophsyiol. 1994 17(11 Pt 2):1974-9). Stimulation of multiple sites within a heart chamber has also been found effective in controlling arrhythmogenic depolarizations (Ref: Ramdat-Misier A., et al., “Multisite or alternate site pacing for the prevention of atrial fibrillation,” Am. J. Cardiol., 1999 11;83(5b):237D-240D).
In order to achieve multi-chamber or multi-site stimulation in a clinical setting, conventional dual-chamber pacemakers have now been used in conjunction with adapters that couple together two leads going to different pacing sites or heart chambers. Reference is made Cazeau et al. (Pacing Clin. Electrophsyiol. 1994 17(11 Pt 2):1974-9) that describes a four chamber pacing system in which unipolar right and left atrial leads are connected via a bifurcated bipolar adapter to the atrial port of a bipolar dual chamber pacemaker. Likewise, unipolar right and left ventricular leads are connected via a bifurcated bipolar adapter to the ventricular channel. The left chamber leads were connected to the anode terminals and the right chamber leads were connected to the cathode terminals of the dual chamber device. In this way, simultaneous bi-atrial or simultaneous bi-ventricular pacing is achieved via bipolar stimulation but with several limitations.
One limitation of the multi-chamber stimulation systems described above is that simultaneous stimulation of left and right chambers, as required when two leads are coupled together by one adapter, is not always necessary or desirable. For example, in some patients conduction between the two atria may be compromised, however the pacemaking function of the sinus node in the right atrium may still be normal. Hence, detection of an intrinsic depolarization in the right atrium could be used to trigger delivery of a pacing pulse in the left atrium. Since an intrinsic depolarization has occurred in one chamber, simultaneous stimulation of both chambers in this situation is unnecessary.
In another example, when inter-atrial or inter-ventricular conduction is intact, stimulation in one chamber may be conducted naturally to depolarize the second chamber. A stimulation pulse delivered in one chamber, using the minimum energy required to depolarize that chamber, will be conducted to the opposing chamber thus depolarizing both chambers. In this case, stimulation of both chambers simultaneously would be wasteful of battery energy.
In the presence of an inter-atrial or inter-ventricular conduction defect, one may want to control the interval between a sensed or paced event in one chamber and delivery of a stimulation pulse to the other chamber. If pacing is required in both the left and right chambers, the control of the sequence of the stimulation pulse delivery to each chamber, rather than the simultaneous delivery of stimulation pulses, may be necessary in order to achieve a specific activation sequence that has hemodynamic benefit.
Even in patients with intact conduction, it may be desirable to precisely control the activation sequence of the heart chambers in order to provide maximum hemodynamic benefit. Precise control of the activation sequence may improve the coordination of heart chamber contractions resulting in more effective filling and ejection of blood from the heart.
In certain currently available devices, adapters are no longer required. The connection between leads is hardwired internally in the connector block coupling the ventricular leads to the ventricular channel and the atrial leads to the atrial channel. While this design advantageously eliminates the need for adapters, the hardwire connections preclude the introduction of separate timing between stimulation pulses delivered to each chamber or responding with any programmable delays to a sensed event by delivery of an output pulse to the other chamber.
To address some of these limitations, multichamber stimulation devices have been proposed that allow some degree of independent stimulation and/or sensing of different chambers of the heart. Reference is made, for example, to U.S. Pat. No. 5,720,768 to Verboven-Nelissen, U.S. Pat. 5,902,324 to Thompson et al., U.S. Pat. No. 6,081,748 to Struble et al.
To further enhance the benefit provided to the patient during periods of exercise or increased metabolic demand, it would be desirable to allow the inter-chamber stimulation intervals to be automatically adjusted as changes in stimulation rate occur in response to a physiological sensor of metabolic need, or in response to algorithmic adjustments. Therefore, what is needed is a multi-chamber stimulation device that provides programmable inter-chamber timing intervals that are also automatically adjusted as a function of the changing stimulation rate.